Does your furnace keep shutting off unexpectedly or running in shorter cycles? If so, you might have a problem with your furnace limit switch. Also known as a high limit switch, this small (but critical) component helps keep your home safe by automatically shutting off the system when your furnace gets too hot. In this guide, we’ll explain how your furnace high limit switch works, how it impacts your home, and solutions for when it malfunctions.
What is a furnace limit switch?
A furnace limit switch is a device within your furnace that detects when the internal temperature is too high and turns the unit off as a safety measure. You may also hear it referred to as a fan limit switch or a limit switch.
When your furnace is working correctly, you shouldn’t have to worry about your furnace limit switch. But when it starts to trip or malfunction, you may need to know about this part and how to fix it.
What does a furnace limit switch do?
Let’s get into a more technical explanation of what your furnace limit switch does in the context of the entire heating system. When you turn up the heat on your thermostat, the burners will activate and begin heating up air inside the plenum.
When the unit is at an appropriate temperature, the limit switch in the furnace allows for burner operation and the furnace will continue to send out heat. If the temperature gets too high within the furnace, the switch will open, causing the furnace to stop operating. The burners will shut down and the fan will continue to operate, cooling down the internal components of the furnace.
When the furnace’s internal temperature decreases, the limit switch will close, allowing the furnace to start back up for normal operation.
How a furnace limit switch impacts your safety and comfort
Since your furnace uses gas to heat your home, it must have safety features in place to prevent it from overheating or breaking. If those safety components fail, necessary parts of the furnace, like the heat exchanger, can break more easily.
Fortunately, your switch acts as a safety device to control these variables and keep your family safe. It does this by shutting off power to the gas valve when the furnace exceeds the manufacturer’s temperature limit. If a gas furnace warms your home, install a carbon monoxide alarm to detect the odorless gas in case of a leak.
What does a furnace limit switch look like?
A furnace limit switch has two components. The first is a small button-shaped mounting plate that’s fastened outside of the furnace plenum. It attaches to a long temperature probe that extends through the furnace housing.
Where is the limit switch on a furnace?
In most cases, the limit switch will be located inside the cover panel of your furnace. (You might need a screwdriver to remove this panel, depending on the model of your system.) On older furnaces, however, it may be set up on the outside of your furnace.
Furnace limit switch tripping
Because the limit switch is designed to detect and shut down issues within your furnace, there are several reasons that it might trip. For example, if your furnace keeps shutting off soon after you fire up your thermostat, it could be indicative of issues such as:
- Airflow problems inside your furnace (including dirty air filters), which cause overheating
- Dirty temperature sensor on your furnace limit switch
- Defective high limit switch
In order to avoid these issues, it’s important to keep up with the recommended maintenance schedule from your furnace manufacturer. For example, many manufacturers advise you to change your furnace filter every three months.
When necessary, you should also clean the temperature sensor on your limit switch to keep it in good working condition and prevent tripping. If your switch continues tripping after you’ve replaced your filter and cleaned the sensor probe, you may have a defective limit switch (more on that below).
When your switch repeatedly trips, it may enter a “lockout mode,” where the furnace will completely shut down. This can only be reversed by servicing and resetting the furnace, which should be done by a professional.
How do I know if my limit switch is bad?
As we’ve covered, your limit switch can signal other issues with your furnace that aren’t necessarily related to the switch itself. So how do you know if it’s your switch that’s the problem? Here are some warning signs.
- Furnace blowers don’t shut off
- Air coming out of your system is cool
- System repeatedly turns on and off (called “short cycling”)
How do you reset a furnace limit switch?
If your furnace shuts down several times in a row, it may enter a hard lockout mode that will require you to reset your switch or the entire furnace system. This happens as a safety mechanism to prevent your furnace from cracking and leaking carbon monoxide into your home.
You may be able to reset the system yourself, depending on the age and model of your furnace. If you have a newer model, you should be able to turn off the power for about 30 seconds and then turn it back on.
If you can’t reset it yourself or if it repeatedly goes into hard shutdown, it’s a good idea to contact a specialist and have them perform a safety check. A faulty furnace can have life-threatening consequences for your family, so it’s essential to confirm that everything is operating correctly.
Can I replace a furnace limit switch myself?
Yes, it’s possible to replace a furnace limit switch yourself. To do it, you’ll first need to use a multimeter to test the switch. If it’s faulty, you’ll also need a new switch that matches your old one. Here’s how to test the furnace limit switch and replace it if necessary.
- Shut off the power and gas to your furnace.
- Locate the limit switch and remove it from your furnace.
- Place the multimeter probes on the switch terminals and check for continuity.
- If there’s no continuity, replace the switch with a new model.
Before trying to complete any work yourself, keep in mind that furnaces can be dangerous. Not only do they emit hot air, but they also contain flammable gas and run on high-voltage electricity. For those reasons, you shouldn’t try to complete any complicated repairs on your fan limit switch or your furnace unless you know what you’re doing. If not, call in an HVAC specialist for help.
Furnace limit switch repair and replacement costs
If you’re the DIY type, you’ll save quite a bit of money by replacing your furnace limit switch yourself (assuming you already have a multimeter at home). You can find new switches for less than $20 at your local hardware store. However, if replacing the switch doesn’t solve your issue, you should contact an HVAC expert.
The cost of professionally repairing your furnace will vary depending on the specific issue. Generally speaking, you should expect to pay between $100 and $1,000 per furnace repair visit, with an average of $270 per repair.
When to hire furnace repair specialist
If you’re still having problems with your furnace after replacing the switch, changing the air filter, and cleaning the temperature sensor, it may be time to replace the entire furnace. You can use the HVAC.com cost calculator to estimate the price of replacing your furnace and connect with trusted providers in your area.
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